EFFECT OF SOIL CONDITIONS ON SEISMIC RESPONSE OF A BRIDGE

Abstract

This Dissertation is concerned with study of effect of soil conditions on seismic response of a bridge. This study is mainly focused on the behavior of a bridge on four different soil sites, namely, La Cienega (Los Angeles), KGWI-102 (Japan), Lotung (Taiwan) and Treasure Island near San Francisco bay area categorized as NEHRP site class D. Current design codes classif' soil sites in different classes based on shear wave velocity or standard penetration test value. The difference in response of structure on different soil sites under same site class is studied. The primary aim of the present study is to examine the effects of foundation-soil flexibility and site amplification on response of the structure. A three span box girder integral bridge is considered with pier height of 10 m. Formulations to model layered soil is discussed. Modelling of soil -foundation flexibility for layered soil in lateral and vertical direction is presented. Different methods of scaling time histories are presented. The performance of bridge with foundation-soil flexibility is evaluated under seismic conditions for rock outcrop motion and site surface motion to study the effect of site amplification on seismic response of a bridge. Bridge modeled on four sites shows elastic behavior when subjected to rock outcrop motion. When subjected to site surface motion, the response of bridge shows large difference. In case of KWGH02 site and Treasure Island site yielding of structure is observed, whereas in case of La-Cienega, yielding of soil is observed. Bridge modeled on Lotung site shows elastic behavior. The performance of bridge is evaluated with fixed base and foundation —soil flexibility, when subjected to rock outcrop motion to study the effect of foundation flexibility on seismic response of a bridge. It is found that there is significant difference in the stiffness of the structure and the response of structure due to foundation flexibility. Also. a comparison of results from non-linear static and dynamic analysis is presented in this study. It is found that the results have considerable difference.